Crude oils from which desired gaseous and liquid fuels are made contain a diverse mixture of hydrocarbons, sulfur and nitrogen compound and metal contaminants. The hydrocarbon compounds vary widely in molecular weight and therefore boil over a wide range. For example, crude oils are known in which 30 to 60% or more of the total volume of oil is composed of compounds boiling at temperatures above 650.degree. F. Among these are crudes in which from about 10% to about 30% or more of the total volume consists of compounds so heavy in molecular weight that they boil above 1025.degree. F. or at least will not boil below 1025.degree. F. at atmospheric pressure.
Because these high boiling components of crude oil boiling above 650.degree. F. are unsuitable for inclusion in gasoline and some higher boiling liquid hydrocarbon fuels, the petroleum refining industry has developed processes for separating and/or breaking the molecules of the high molecular weight, high boiling compounds into smaller molecules which do boil over an appropriate boiling range. The cracking process which is most widely used for this purpose is known as fluid catalytic cracking (FCC). Although the FCC process has reached a highly advanced state, and many modified forms and variations have been developed, their unifying factor is that a very restricted boiling range hydrocarbon feedstock is preferably caused to be cracked at an elevated temperature in contact with a cracking catalyst that is suspended in the feedstock under cracking conditions in a temperature range of 950.degree. to about 1100.degree. F. Upon attainment of a desired degree of molecular weight and boiling point reduction the cracking catalyst is separated from the obtained hydrocarbon vaporous conversion products.
Crude oils in the natural state contain a variety of materials which tend to have quite troublesome effects on FCC processes; and only a portion of these troublesome materials can be economically removed from the crude oil. Among these troublesome materials are coke precursors (such as asphaltenes, polynuclear aromatics, etc.), heavy materials (such as nickel, vanadium, iron, copper, etc.), lighter metals (such as sodium, potassium, etc.), sulfur, nitrogen and others. Certain of these, such as the lighter metals, can be economically removed by desalting operations, which are part of the normal procedure for pretreating crude oil for fluid catalytic cracking. Other materials, such as coke precursors, asphaltenes and the like, tend to break down into coke during the cracking operation, which coke deposits on the catalyst, impairing contact between the hydrocarbon feedstock and the catalyst, and generally reducing its potency or activity level. The heavy metals transfer almost quantitatively from the feedstock to the catalyst surface.
If the catalyst is reused again and again for processing additional feedstock, which is usually the case, heavy metals in the feedstock can accumulate on the catalyst to the point that they unfavorably alter the composition of the catalyst and/or the nature of its catalytic effect upon the feedstock. For example, vanadium tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles sufficiently so that they begin to sinter and become ineffective cracking catalysts. Accumulations of vanadium and other heavy materials, especially nickel, on the catalyst act as a poison in some environments. They tend in varying degrees to promote excessive dehydrogenation and aromatic condensation, resulting in excessive production of carbon and gases with consequent impairment of liquid fuel yield. An oil such as a crude or a high boiling crude fraction or other source oil that is particularly abundant in metal contaminants such as nickel and/or other metals exhibiting similar behavior, while containing relatively large quantities of coke precursors, is referred to herein as one comprising metallo-organic compounds or as a carbometallic oil, and represents a particular processing challenge to the petroleum refiner to yield desired products such as gasoline, gasoline precursors and light cycle oils.
There has been a long standing interest in the catalytic conversion of carbometallic oils into gasoline and other liquid fuels. For example, in the 1950's it was suggested that a variety of carbometallic oils could be successfully converted to gasoline and other products in the Houdresid process. The Houdresid process employed catalyst particles of "granular size" (much larger than conventional FCC catalyst particle size) in a compact gravitating bed, rather than suspending catalyst particles in feed and product vapors in a fluidized catalyst bed system. The productivity of the process, compared to fluid catalytic cracking with lighter gas oils, was low. But the Houdresid process did offer some advantages. It appeared that the adverse effects previously encountered with heavy metals in the feed were not as great a barrier in the Houdresid process as one might expect in FCC processing. The heavy metal which accumulated on or near the outer surfaces of the catalyst particles apparently could be removed to some extent by an attrition process, which selectively removed an outer layer of metal-contaminated catalyst. The catalysts were very cheap, but also relatively inactive, and highly unsuitable by today's standards. While the maximum tolerable limit of heavy metal contamination on catalyst in fluid catalytic cracking was then thought to be about 200 parts per million, the Houdresid process did continue to operate satisfactorily even when the total nickel plus vanadium content of the catalyst bed reached 870 ppm. Moreover, it was found that the required levels of product selectivity could be maintained without withdrawing catalyst from the process, except to the extent that withdrawal was required by normal mechanical losses (e.g. attrition and inadvertent discharge with off gases) and by the attrition and inadvertent discharge with off gases) and by the attrition method used to control metals level. Today, such attrition of catalyst to fine particulates would present an expensive environmental problem, thus considerably increasing difficulties involved in practicing the process.
Although the Houdresid process obviously represented a step forward in dealing with the effects of metal contamination and coke formation on catalyst performance, its productivity was limited. Thus, for the many years which have passed since the Houdresid process was first introduced commercially, the FCC process art has continued an arduous search for suitable modifications or alternatives to the FCC process which would permit commercially successful operation on reduced crude and the like. During this operating and search period a number of proposals have been made; some have been used commercially to a certain limited extent.
Several proposals involve treating a heavy or high boiling oil feed to remove the metal therefrom prior to cracking, such as by hydrotreating, solvent extraction and/or complexing with Friedel-Crafts catalysts, but these techniques have been criticized as unjustified economically. Another proposal employs a combination cracking process having "dirty oil" and "clean oil" processing units. Still another proposal blends residual oil with gas oil and controls the quantity of residual oil in the mixture in relation to the equilibrium flash vaporization temperature at the bottom of the riser type cracker unit employed in the process. Still another proposal subjects the feed to a mild preliminary hydrocracking or hydrotreating operation before it is introduced into the cracking unit. It has also been suggested to contact a carbometallic oil such as reduced crude with hot taconite pellets to produce gasoline. This is only a small sampling of many proposals which have appeared in the patent literature and technical reports for upgrading carbometallic oil feeds.
Notwithstanding the great effort which has been expended and the fact that each of these proposals overcomes some of the difficulties involved, conventional FCC practice today bears mute testimony to the dearth of carbometallic oil-cracking techniques that are both economical and highly practical in terms of technical feasibility. Some crude oils are relatively free of substantial coke precursors or heavy metal contaminants or both, and the most troublesome components of crude oil are for the most part concentrated in the highest boiling fractions thereof. It has been possible in the prior art to largely avoid the problems of coke precursors and heavy metals by sacrificing the liquid fuel yield which would be potentially available from the highest boiling fractions. More particularly, conventional FCC practice has employed as a part of the gas oil feedstock that fraction of crude oil which boils at about 650.degree. F. to about 1000.degree. F., such fractions being relatively free of heavy metal contamination. Such feedstock, known as "vacuum gas oil" (VGO) is generally prepared from crude oil by distilling off the fractions boiling below about 650.degree. F. at about atmospheric pressure and then separating by further vacuum distillation from the heavier fractions a vacuum gas oil cut boiling from 650.degree. F. up to about 900.degree. F. or 1025.degree. F.
A gas oil of atmospheric distillation in combination with vacuum gas oil is most often used as feedstock for prior art conventional FCC processing. The heavier fractions of the crude oil are normally employed for a variety of other purposes, such as for instance production of asphalt, residual fuel oil, #6 fuel oil, or marine Bunker C fuel oil, which represents a great waste of the potential value of this portion of the crude oil. This is especially true in light of the great effort and expense which the art has been willing to expend in the attempt to produce generally more suitable feed materials from coal and shale oils.
The present invention is particularly aimed at the cracking of gas oils with or without the heavier fractions of crude oils and particularly that portion thereof containing substantial quantities of both coke precursors, heavy metals, and other troublesome components either alone or in conjunction with the lighter more suitable oils, thereby potentially increasing the overall yield of gasoline and other desired gaseous and liquid fuels from a given crude oil. It is believed that the method and means comprising the process of this invention is uniquely advantageous for dealing with the problem of treating gas oils with and without high boiling carbometallic oils in an economically and technically sound manner.
In general, the coke-forming tendency or coke precursor content of an oil can be ascertained by determining the weight percent of the carbon remaining after a sample of that oil has been pyrolized. The industry accepts this value as a measure of the extent to which a given oil tends to form non-catalytic coke when employed as feedstock in a catalytic cracker. Two established tests are recognized, the Conradson Carbon and Ramsbottom Carbon Tests, the latter being described in ASTM Test No. D524-76. In conventional FCC practice, Ramsbottom carbon values on the order of about 0.1 to about 1.0 are regarded as indicative of acceptable feed. The present invention is concerned with the use of hydro-carbon feedstocks which have higher Ramsbottom carbon values and thus exhibit substantially greater potential for coke formation than the usual feeds.
Since the various heavy metals are not of equal catalyst poisoning activity, it is convenient to express the poisoning activity of an oil containing a given poisoning metal or metals in terms of the amount of a single metal which is estimated to have equivalent poisoning activity. Thus, the heavy metals content of an oil can be expressed by the following formula (patterned after that of W. L. Nelson in Oil and Gas Journal, page 143, Oct. 23, 1961) in which the content of each metal present is expressed in parts per million of such metal, as metal, on a weight basis, based on the weight of feed. ##EQU1##
According to conventional FCC practice, the heavy metal content of a feedstock for FCC processing is controlled at a relatively low level, e.g. about 0.25 ppm Nickel Equivalents or less. The present invention is concerned with the processing of feedstocks containing metal contaminants substantially in excess of this value, and which therefore have a significantly greater potential for accumulating on and poisoning catalyst.
The above formula can also be employed as a measure of the accumulation of heavy metals on cracking catalyst, except that the quantity of metal employed in the formula is based on the weight of catalyst (moisture free basis) instead of the weight of feed. In some known FCC prior art processes, in which a circulating inventory of catalyst is used again and again in the processing of fresh feed, with periodic and/or continuing minor addition and withdrawal of fresh and spent catalyst, the metal content of the catalyst is maintained at a level which may for example be in the range of about 200 to about 6000 ppm Nickel Equivalents. The process of the present invention is concerned with the use of catalyst having an accumulated large metals content, and which therefore has the potential in the absence of controls to promote dehydrogenation, aromatic condensation, gas production or coke formation.